1. Field of the Invention
This invention is related to the field of packages for Microwave Circuits, especially in the frequency band from 20 GHz and higher.
2. Prior Art
Microwave Monolithic Integrated Circuits (MMIC) and other microwave integrated circuit (IC) devices for microwave signals have been packaged using complex, high reliability, high cost packages, the designs of which are not conducive to high volume assembly. These MMIC packages (packages) have been used in such specialty microwave applications as Radio Frequency (RF) telescopes and other speciality applications.
However, in the growing markets of more mainstream commercial applications, e.g., Personal Communication Services, there is the need for low cost, wideband, surface mounted, reliable, user friendly MMIC packages. One such example of a growing commercial microwave application is the development of Line Microwave Distribution Systems (LMDS), two point radios and obstacle detection radar for automotive vehicles. Moreover other applications have been identified at 23, 28, 32, 38, 60 and 70 Ghz and higher, and the industry is approaching production of these products.
The MMIC packages are intended primarily for MMICs constructed substantially of semiconductor materials suitable for high frequency operation, e.g, Gallium Arsenide. The MMIC is generally bonded to a package substrate constructed of a ceramic material, e.g., alumina, having thin films of metals, e.g., nickel or titanium, disposed on its surface in the form of an arrangement of wave guides. The waveguides comprise a patterned metalization which includes a plurality of ground planes, signal traces and unmetalized gaps through which the microwave signals are transmitted. The MMIC is connected to the ground planes and signal traces on the surface of the substrate by methods such as wirebonding, solder bumps or flipped chips. Vias, e.g., holes or slots, in the ceramic substrate are filled with a conductive composite material such as copper-tungsten, which provides a transmission path for the signal from the metalization on the surface through the substrate itself. The composite material within the vias is in turn connected to input/output (I/O) ports of the MMIC package, which transmit the microwave signals therethrough. Existing packages are surface mounted to a mother PC board and are connected to the circuitry of the PC board through the I/O ports, which can include conductive balls (spheres), bumps (rounded protrusions), ribbons or leads.
The MMIC of the MMIC package is connected to the mother PC board in a way that is electrically transparent to microwave signals transmitted therebetween. Achieving electrical transparency requires matching the impedance of the microwave transmission path through all transition areas for the signal, i.e., areas between waveguides, from the mother board to the MMIC. Prior art transition areas for the signal include: 1) from the mother board through the I/O connection between the mother board and the package substrate, 2) through the substrate itself, and 3) from the surface where the MMIC is installed through the connection (e.g. wirebond, flip chip) to the MMIC.
Referring to FIG. 1, a prior art MMIC package 10 is illustrated where a mother PC board 12 has a waveguide 14 bonded to its top surface, and a substrate 16 has waveguides 18 and 20 bonded to its underside and topside respectively. The waveguide 14 includes signal conductor 30 and ground conductors 32 and 34; waveguide 18 includes signal conductor 36 and ground conductors 38 and 40; and waveguide 20 includes signal conductor 42 and ground conductors 44 and 46. Transition area 1, i.e., I/O ports of package 10, comprises thin and wide conductive stubs 22, transition area 2, i.e., through substrate 16, comprises narrow and wide slots (vias) 24 in the package substrate filled with an electrically conductive material, and transition area 3 comprises wire bonds 26 connected to the MMIC 28. FIG. 1 illustrates where the dielectric constants of the substrate 16 and the mother board 12 are the same and the two transitions 1 and 2 of the waveguide structures are very close to the ideal. One of the factors in the representation of FIG. 1 that makes the transitions close to ideal are the conductively filled vias 24 and stubs 22 that match the width of the conductors with which they make contact (30, 32, 34 on the mother board; 36, 38, 40 on the underside of the package; and 42, 44 and 46 on the top surface of the substrate 16). The lack of variation in dielectric constant in this Figure enables the use of straight coplanar waveguide structures.
In order to provide the transmission path through the substrate 16 itself, the transition area 2, the vias 24 are preferably drilled in the ceramic substrate with a carbon dioxide or YAG laser. The slots are then filled with the conductive composite material in a powder form, whereby the powder is melted in ovens and then cooled to provide conductively filled through structures, i.e., vias, for the microwave signals. However the ceramic substrate is brittle and can often be cracked by the thermal expansion and contraction of the composite material. This is especially problematic when there are a large number of vias for a complex circuit or the substrate is relatively thin.
Problematically, package substrate thickness is defined by a tradeoff between performance, mechanical strength and cost. The thicker the substrate the greater the mechanical strength and the less breakage that occurs during the manufacturing process. Yet it is generally required to minimize via length, by reducing the thickness of the substrate, in order to enhance high frequency performance and to facilitate impedance matching of the transition area. However, the brittle ceramic substrate can contain a high density of vias, which makes the substrate very fragile, difficult to handle and expensive. Therefore, the chances of losses due to thin substrate breakage are increased when the substrate is made thinner. Thus, the requirement for a thinner substrate may result in losses due to substrate breakage.
The deficiencies and limitations of the above package are eliminated or greatly alleviated by the present invention.
This invention offers advantages and alternatives over the prior art by providing a circuit package for a microwave signal having a reduced number of vias in the substrate. The substrate of the package is devoid of signal carrying vias. Advantageously, the invention eliminates a transition area for the signal through the substrate of the package. Moreover, because signal carrying vias are avoided the substrate can be made substantially thick so that manufacturing yield losses due to substrate breakage can be virtually eliminated. Additionally, high frequency performance is enhanced and impedance matching is facilitated.
These and other advantages are accomplished in an exemplary embodiment of the invention by providing a circuit package for a microwave signal. The circuit package comprises a substrate defining a MMIC surface of the substrate and an opposing non-MMIC surface of the substrate. The substrate is devoid of signal carrying vias. An arrangement of waveguides are disposed on the MMIC surface of the substrate. A MMIC is disposed on the MMIC surface of the substrate, and the MMIC is in electrical communication with the waveguide(s). An I/O port is in electrical communication with the waveguide wherein a transmission path for the signal is provided from the I/O port, through the waveguide and to the MMIC.
In a preferred embodiment of the invention, the MMIC surface of the substrate faces the PC board when the I/O port is electrically connected to the PC board.
In another exemplary embodiment, the circuit package includes at least one ground carrying via providing a conductive connection between a pair of ground planes for the signal. The pair of ground planes are disposed on the MMIC surface and the opposing non-MMIC surface of the substrate.
In yet another preferred embodiment of the invention, signal vias continue to be avoided but thermal vias are provided extending from the MMIC surface of the substrate to the non-MMIC surface of the substrate. The thermal vias are arranged to be in thermally conductive contact with the MMIC on one end and a heat sink attached to the non-MMIC surface of the substrate on the opposite end. Since the MMIC surface of the substrate faces the PC board the heat sink is located in a more unrestricted environment for air passage and easily dissipates heat to the environment.
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.